Don't bend too far in the positive-attitude direction by using language that is so lofty that it comes off sounding almost phony, as in this example:

Dear Mrs. Linville:
I am writing in response to the employment advertisement of juvenile care worker as listed in The County Leader. I share a deep concern and commitment to humanity, especially our children.

After reviewing the enclosed resume, you will be convinced that interviewing me will be mutually beneficial. I am anxiously awaiting the opportunity to fulfill my responsibility to all humankind.